1. Field of the invention
The present invention relates to a semiconductor device manufacturing apparatus, and more specifically an electroless plating or electroplating apparatus used in the process of manufacturing semiconductor devices.
2. Description of Related Art
In recent VLSIs (very large scale integrated circuits), a study has been made of using a noble metal as a wiring material from a viewpoint of reliability and performance. In particular, attention has been focused on gold (Au) as the wiring material, since Au has a low specific resistance and is extremely chemically stable.
Generally, in the case of forming an Au wiring conductor, a method of using a photolithography and a gold plating technique in combination for forming a desired wiring pattern has been widely used.
The gold plating technique can be divided into an electroplating and an electroless plating, and also can be divided into a so-called "dipping method" in which a whole of a semiconductor wafer (substrate) is dipped into a treating tank, and a so-called "face-down method" in which a semiconductor wafer is faced down so that only a faced-down surface of the semiconductor wafer is brought into contact with a treating liquid. However, the dipping method is disadvantageous in which soils and dusts on a rear surface of the semiconductor wafer contaminates the plating liquid, with the result that quality of a plated film is deteriorated, and Au is deposited on the rear surface of the semiconductor wafer to some extent. This is a cause of so called "particles". Therefore, in the LSIs in which an extremely fine pattern has to be realized, the face-down method has been used for a mass production.
Referring to FIG. 1, there is illustrated an example of a conventional face-down method, A plating tank 30 is located upwardly and has a cathode 31 provided along an upper directed opening thereof. The plating tank also includes a mesh anode 32 located at a meddle level thereof positioned to oppose the opening of the plating tank. A semiconductor wafer 1 is supported at its periphery by the cathode 31 in such a manner that a front surface of the semiconductor wafer 1 to be treated is faced down so as to oppose the mesh anode 32 located at the middle level of the plating tank 30. With this arrangement, a DC or pulsed current is applied from a power supply 33 between the cathode 31 and the anode 32, while a plating liquid 34 is continuously introduced from a bottom of the plating tank 34 in a direction of an arrow D toward the front surface of the semiconductor wafer 1 so that the plating liquid 34 overflows from the upper edge of the plating tank 30 in directions of arrows B and C.
In the above mentioned plating apparatus, a maximum problem is a countermeasure for bubbles. An optimum operation temperature of commercially available gold plating liquid is ordinarily 60.degree. C. to 75.degree. C., and a good film quality can be obtained at about 70.degree. C. At this temperature, air dissolved in the plating liquid gasifies into the form of extremely small bubbles and adheres on a wall of a piping conduit and the tank, and when the bubbles grow to a some degree of size, the bubbles depart from the wall so as to move together with the plating liquid. As illustrated in FIG. 2, if the bubble(s) reaches the front surface of the semiconductor wafer together with the plating liquid, the bubble 36 adheres a stepped or concave portion of a patterned photoresist film 35 deposited on the semiconductor wafer, with the result that an uneven plated film is formed.
In order to overcome this unevenness of the plated film, there having been proposed various methods, which include a method of greatly increasing the flow rate of the plating liquid so that the bubble 36 is blown out by the forced plating liquid flow, another method for moving up and down the liquid surface of the plating liquid, and a third method of forming a plurality of holes for injecting the plating liquid into the plating tank so as to ceaselessly change the hole of injecting the plating liquid 34, so that the position of of injecting the plating liquid 34 is changed. However, any of the proposed methods is not satisfactory, since the bubble 36 is difficult to be removed because all the proposed methods are basically of the face-down method.
Specifically, in the method of blowing out the bubble 36 by the forced plating liquid flow, the plating liquid 34 becomes easy to scatter from the upper opening of the plating tank 30, and therefore, the circumstance of the plating tank becomes dirty, and splashes of the plating liquid 34 is deposited on the rear surface of the semiconductor wafer. Namely, the splashes of the plating liquid 34 become a cause of the soil 0f the rear surface of the semiconductor wafer.
In addition, the method of moving up and down the liquid surface is disadvantageous in that when the liquid surface is lowered, the wafer surface is dried, and therefore, deposition occurs, which becomes a cause for the uneven plating and the soil.
The method of changing the position of of injecting the plating liquid 34 has various defects. For example, the structure of the apparatus becomes complicated.
Under the above mentioned circumstance, at present, it has been the most practical bast way to maintain the temperature of the plating liquid 34 not greater than 60.degree. C. so as to suppress the generation of the bubbles 36. However, if the temperature of the plating liquid 34 is lowered to a temperature not greater than 60.degree. C., unevenness occurs in the quality of the plated film, with the result that the lift of the obtained wiring conductors becomes unstable, and an appearance of the surface also becomes unstable.
On the other hand, the bubbles are generated by another reason, which becomes a fatal defect particularly in the case of the electroless plating. The plating is a reduction reaction, regardless of whether it is the electroplating or the electroless plating. Therefore, a gas of H.sub.2 is generated in the course of the reaction. In particular, this gas is generated at the wafer surface, and therefore, even if the temperature of the plating liquid 34 is lowered, the generation of the bubbles cannot be completely suppressed. In the electroless plating, since a time required for obtaining the same plate film thickness is several times to several tens times that required in the electroplating, the influence of the H.sub.2 gas is remarkable.